1.\" Copyright (c) 1995 Paul Kranenburg 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgment: 14.\" This product includes software developed by Paul Kranenburg. 15.\" 3. The name of the author may not be used to endorse or promote products 16.\" derived from this software without specific prior written permission 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 22.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 23.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 24.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 25.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 26.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 27.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 28.\" 29.\" $FreeBSD$ 30.\" 31.Dd May 6, 2012 32.Dt RTLD 1 33.Os 34.Sh NAME 35.Nm ld-elf.so.2 , 36.Nm ld-elf.so.1 , 37.Nm rtld , 38.Nm _rtld_functrace 39.Nd run-time link-editor 40.Sh SYNOPSIS 41.Ft int 42.Fn _rtld_functrace "const char *callerso" "const char *calleeso" "const char *calleefun" "void *stack" 43.Sh DESCRIPTION 44The 45.Nm 46utility is a self-contained shared object providing run-time 47support for loading and link-editing shared objects into a process' 48address space. 49It is also commonly known as the dynamic linker. 50It uses the data structures 51contained within dynamically linked programs to determine which shared 52libraries are needed and loads them using the 53.Xr mmap 2 54system call. 55.Pp 56After all shared libraries have been successfully loaded, 57.Nm 58proceeds to resolve external references from both the main program and 59all objects loaded. 60A mechanism is provided for initialization routines 61to be called on a per-object basis, giving a shared object an opportunity 62to perform any extra set-up before execution of the program proper begins. 63This is useful for C++ libraries that contain static constructors. 64.Pp 65When resolving dependencies for the loaded objects, 66.Nm 67may be allowed to translate dynamic token strings in rpath and soname 68by setting 69.Fl "z origin" 70option of the static linker 71.Xr ld 1 . 72The following strings are recognized now: 73.Bl -tag -width ".Pa $PLATFORM" 74.It Pa $ORIGIN 75Translated to the full path of the loaded object. 76.It Pa $OSNAME 77Translated to the name of the operating system implementation. 78.It Pa $OSREL 79Translated to the release level of the operating system. 80.It Pa $PLATFORM 81Translated to the machine hardware platform. 82.El 83.Pp 84The 85.Nm 86utility itself is loaded by the kernel together with any dynamically-linked 87program that is to be executed. 88The kernel transfers control to the 89dynamic linker. 90After the dynamic linker has finished loading, 91relocating, and initializing the program and its required shared 92objects, it transfers control to the entry point of the program. 93The following search order is used to locate required shared objects: 94.Pp 95.Bl -enum -offset indent -compact 96.It 97.Dv DT_RPATH 98of the referencing object unless that object also contains a 99.Dv DT_RUNPATH 100tag 101.It 102.Dv DT_RPATH 103of the program unless the referencing object contains a 104.Dv DT_RUNPATH 105tag 106.It 107Path indicated by 108.Ev LD_LIBRARY_PATH 109environment variable 110.It 111.Dv DT_RUNPATH 112of the referencing object 113.It 114Hints file produced by the 115.Xr ldconfig 8 116utility 117.It 118The 119.Pa /usr/lib 120directory unless the referencing object was linked using the 121.Dq Fl z Ar nodefaultlib 122option 123.El 124.Pp 125The 126.Nm 127utility 128recognizes a number of environment variables that can be used to modify 129its behaviour as follows: 130.Bl -tag -width ".Ev LD_LIBMAP_DISABLE" 131.It Ev LD_DUMP_REL_POST 132If set, 133.Nm 134will print a table containing all relocations after symbol 135binding and relocation. 136.It Ev LD_DUMP_REL_PRE 137If set, 138.Nm 139will print a table containing all relocations before symbol 140binding and relocation. 141.It Ev LD_LIBMAP 142A library replacement list in the same format as 143.Xr libmap.conf 5 . 144For convenience, the characters 145.Ql = 146and 147.Ql \&, 148can be used instead of a space and a newline. 149This variable is parsed after 150.Xr libmap.conf 5 , 151and will override its entries. 152This variable is unset for set-user-ID and set-group-ID programs. 153.It Ev LD_LIBMAP_DISABLE 154If set, disables the use of 155.Xr libmap.conf 5 156and 157.Ev LD_LIBMAP . 158This variable is unset for set-user-ID and set-group-ID programs. 159.It Ev LD_ELF_HINTS_PATH 160This variable will override the default location of 161.Dq hints 162file. 163This variable is unset for set-user-ID and set-group-ID programs. 164.It Ev LD_LIBRARY_PATH 165A colon separated list of directories, overriding the default search path 166for shared libraries. 167This variable is unset for set-user-ID and set-group-ID programs. 168.It Ev LD_PRELOAD 169A list of shared libraries, separated by colons and/or white space, 170to be linked in before any 171other shared libraries. 172If the directory is not specified then 173the directories specified by 174.Ev LD_LIBRARY_PATH 175will be searched first 176followed by the set of built-in standard directories. 177This variable is unset for set-user-ID and set-group-ID programs. 178.It Ev LD_BIND_NOW 179When set to a nonempty string, causes 180.Nm 181to relocate all external function calls before starting execution of the 182program. 183Normally, function calls are bound lazily, at the first call 184of each function. 185.Ev LD_BIND_NOW 186increases the start-up time of a program, but it avoids run-time 187surprises caused by unexpectedly undefined functions. 188.It Ev LD_TRACE_LOADED_OBJECTS 189When set to a nonempty string, causes 190.Nm 191to exit after loading the shared objects and printing a summary which includes 192the absolute pathnames of all objects, to standard output. 193.It Ev LD_TRACE_LOADED_OBJECTS_ALL 194When set to a nonempty string, causes 195.Nm 196to expand the summary to indicate which objects caused each object to 197be loaded. 198.It Ev LD_TRACE_LOADED_OBJECTS_FMT1 199.It Ev LD_TRACE_LOADED_OBJECTS_FMT2 200When set, these variables are interpreted as format strings a la 201.Xr printf 3 202to customize the trace output and are used by 203.Xr ldd 1 Ns 's 204.Fl f 205option and allows 206.Xr ldd 1 207to be operated as a filter more conveniently. 208If the dependency name starts with string 209.Pa lib , 210.Ev LD_TRACE_LOADED_OBJECTS_FMT1 211is used, otherwise 212.Ev LD_TRACE_LOADED_OBJECTS_FMT2 213is used. 214The following conversions can be used: 215.Bl -tag -width 4n 216.It Li %a 217The main program's name 218(also known as 219.Dq __progname ) . 220.It Li \&%A 221The value of the environment variable 222.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME . 223Typically used to print both the names of programs and shared libraries 224being inspected using 225.Xr ldd 1 . 226.It Li %o 227The library name. 228.It Li %p 229The full pathname as determined by 230.Nm rtld Ns 's 231library search rules. 232.It Li %x 233The library's load address. 234.El 235.Pp 236Additionally, 237.Ql \en 238and 239.Ql \et 240are recognized and have their usual meaning. 241.It Ev LD_UTRACE 242If set, 243.Nm 244will log events such as the loading and unloading of shared objects via 245.Xr utrace 2 . 246.Pp 247.It Ev LD_LOADFLTR 248If set, 249.Nm 250will process the filtee dependencies of the loaded objects immediately, 251instead of postponing it until required. 252Normally, the filtees are opened at the time of the first symbol resolution 253from the filter object. 254.El 255.Pp 256If a shared object preloaded by the 257.Ev LD_PRELOAD 258mechanism contains a public symbol 259.Dq _rtld_functrace , 260.Nm 261will transfer control to this function each time 262it needs to resolve an unbound function symbol. 263By returning a non-zero value, 264.Fn _rtld_functrace 265can advise the linker to keep tracing the specified 266combination of caller shared object and called function; 267returning 0 will prevent 268.Fn _rtld_functrace 269to be called for this combination again. 270.Pp 271When implementing a custom 272.Fn _rtld_functrace 273function, be aware of the possibility that 274.Fn _rtld_functrace 275might be called for functions called on its behalf, 276or that multiple threads could enter 277.Fn _rtld_functrace 278at the same time. 279.Sh DIFFERENCES BETWEEN .1 and .2 280ABI changes have been made to support TLS allocation and initialization 281and to give threading libraries a chance to complete initialization of the 282TCB prior to the calling of the _init() functions for the dynamically loaded 283libraries. 284.Sh FILES 285.Bl -tag -width ".Pa /var/run/ld-elf.so.hints" -compact 286.It Pa /var/run/ld-elf.so.hints 287Hints file. 288.It Pa /etc/libmap.conf 289The libmap configuration file. 290.El 291.Sh EXAMPLES 292To set up an 293.Fn _rtld_functrace 294for printing out the functions as they are called, this code can be used: 295.Bd -literal -offset indent 296#include <string.h> 297 298static int nl = 10; 299 300int 301_rtld_functrace(const char *callerso, const char *calleeso, 302 const char *calleefun, void *stack) 303{ 304 write(2, "calling ", 8); 305 write(2, calleefun, strlen(calleefun)); 306 write(2, &nl, 1); 307 return 1; 308} 309.Ed 310.Pp 311If put in a file named 312.Pa ft.c 313and compiled with 314.Bd -literal -offset indent 315$ cc -shared -fPIC ft.c -o ft.so 316.Ed 317.Pp 318setting 319.Ev LD_PRELOAD 320to the path of 321.Pa ft.so 322will activate it. 323.Sh SEE ALSO 324.Xr ld 1 , 325.Xr ldd 1 , 326.Xr elf 5 , 327.Xr libmap.conf 5 , 328.Xr ldconfig 8 329